This application is based on application No. H11-305130 filed in Japan on Oct. 27, 1999, the entire contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a laser radiating optical system that shapes a laser beam having an uneven intensity distribution emitted from a light source in such a way as to obtain an even intensity distribution on an irradiation target surface, and in particular to a laser radiating optical system that shapes a laser beam that diverges from a light source in an anisotropic fashion.
2. Description of the Prior Art
Laser beams are characterized by offering high intensity with small beam widths, and are widely used in minuscule machining operations on the surfaces of materials and in data transfer through optical fibers. As light sources that emit laser light, various types are known such as gas lasers as exemplified by carbon dioxide lasers, solid lasers as exemplified by YAG lasers, and semiconductor lasers as exemplified by laser diodes. Any laser light source emits a laser beam that has an uneven intensity distribution, specifically a Gaussian intensity distribution in the single mode.
In machining operations of materials, an object is machined into the shape corresponding to the intensity distribution of the laser beam used. Therefore, the laser beam needs to be controlled so as to exhibit a desired intensity distribution on the object according to the shape into which the object is to be machined. For example, to form a hole having a square cross section and having a uniform depth in the surface of an object, it is necessary to use a laser beam that has a square outline on the sectional plane perpendicular to the optical path and that has an even intensity distribution on that sectional plane. Moreover, the minimum machinable area depends on the beam width of the laser beam used, and therefore it is necessary to give the laser beam a beam width according to the machining precision required. In general, the beam width required in high-precision machining is about a few tens of microns.
When a laser beam is used in data transfer through an optical fiber, which typically has a diameter of a few microns, it is necessary to minimize the loss of laser light by making the beam width still smaller. In addition, an optical fiber has an individual light propagation mode, and a laser beam propagated through an optical fiber has a Gaussian intensity distribution in the single mode. For this reason, if a laser beam emitted from a light source that exhibits a Gaussian intensity distribution is made to have a minuscule beam width simply by being made to converge, it occurs that very little of the laser beam is propagated through the optical fiber when the intensity distribution of the laser beam and the intensity distribution that can be propagated through the optical fiber are out of phase. Moreover, as temperature varies, the degree to which they are out of phase varies, and accordingly the data transferred also varies. This makes it impossible to achieve the desired function.
To prevent this inconvenience, it is necessary to reduce the error in the position at which a laser beam is shone into an optical fiber to about one-tenth of the fiber diameter or less. This requires that the optical fiber and the optical system that directs the laser beam at the optical fiber be positioned precisely relative to each other and that their positions be fixed so as not to vary with temperature. Thus, their alignment requires much time and also a high-precision fixture.
In machining of materials, a laser radiating optical system is used, which is provided with a shaping optical system that makes a laser beam emitted from a light source converge and that converts the intensity of the laser beam in such a way that an even intensity distribution is obtained at the position at which the laser beam converges. Also in data transfer through optical fibers, such a laser radiating optical system can be used so that a laser beam having an even intensity distribution and having a beam width somewhat greater than the fiber diameter is directed to an optical fiber. This makes it possible to keep constant the intensity of the laser beam propagated through the optical fiber even when a small error occurs in the position, relative to the optical fiber, at which the laser beam converges, and is thus expected to make their alignment and fixing easier.
When the aperture through which a light source emits a laser beam is not much different from the wavelength of the laser beam, diffraction occurs in the laser beam traveling out of the aperture and forms the laser beam into a divergent beam that fans out in a conical shape. With gas or solid lasers, the aperture is circular or square, and therefore the laser beam diverges in an isotropic fashion so as to have a substantially circular cross section. Thus, in a laser radiating optical system that employs a gas or solid laser as a light source, it is possible to obtain an even intensity distribution by the use of a shaping element that has an isotropic shaping characteristic.
Usually, to facilitate intensity conversion achieved by a shaping element, a divergent beam emitted from a light source is formed into a parallel beam beforehand by the use of a collimator lens. The shaping element makes the parallel beam converge to make its beam width smaller, and simultaneously converts the intensity distribution of the laser beam so that an even intensity distribution is obtained at the position at which the laser beam converges.
On the other hand, a laser diode has a structure in which a thin active layer is laid between cladding layers so that a laser beam is emitted from a side face of the active layer, and thus has a rectangular aperture. As a result, the angle of diffraction of the laser beam traveling out of the aperture is large in the direction of the shorter sides of the aperture (i.e. in the direction in which the semiconductor layers are laid over one another) and small in the direction of the longer sides thereof. Thus, the laser beam diverges in an anisotropic fashion so as to have an oval cross section. For example, a typical laser diode emits a laser beam whose vertical angle is about 25xc2x0 in the direction of the shorter sides of the aperture and about 10xc2x0 in the direction of the longer sides thereof. Quite naturally, with a laser beam that diverges in an anisotropic fashion, whenever it exhibits a Gaussian intensity distribution, its intensity also is distributed in an anisotropic fashion.
However, in a conventional laser radiating optical system, even when a laser diode is used as a light source, no consideration is given to the fact that the laser beam diverges in an anisotropic fashion. That is, even then, the laser beam is formed into a parallel beam by the use of an isotropic collimator lens, and then its intensity distribution is converted by the use of an isotropic shaping element. As a result, some anisotropy remains even at the position at which the laser beam converges, and thus the range in which an even intensity distribution is obtained differs greatly between two mutually perpendicular directions. Accordingly, in applications where a laser beam having an even intensity distribution is shone onto a region extending equally in two mutually perpendicular directions, only part of the area in which an even intensity distribution is obtained is used, and the remaining part is discarded. Thus, the laser light is utilized with quite low efficiency.
This inconvenience is alleviated by using an anisotropic shaping element instead of an isotropic shaping element. However, the shaping performance of a shaping element that serves to make the intensity distribution of a laser beam even depends heavily on the numerical aperture at the exit side of the shaping element. Therefore, even if an anisotropic shaping element is used, it is extremely difficult to make the intensity distribution satisfactorily even in both of two mutually perpendicular directions. This is because, whereas the beam width of the laser beam entering the shaping element differs between two mutually perpendicular directions, the distance from the shaping element to the convergence position is fixed, and therefore the shaping element cannot have equal numerical apertures in two mutually perpendicular directions.
Now, the relationship between the numerical aperture of a shaping element and the intensity distribution obtained as a result of shaping will be described, taking up an isotropic laser beam as an example. When a laser beam having a comparatively large width and having an intensity distribution as shown in FIG. 10 is made to converge at a convergence position a predetermined distance away from a shaping element as a laser beam having a square cross section and having an even intensity distribution, the obtained intensity distribution is as shown in FIGS. 11A and 11B. In this way, a laser beam having a large width is formed into a laser beam of which the intensity drops abruptly in its peripheral portion and which thus exhibits sharp intensity variation. Accordingly, an even intensity distribution is obtained in a large area, and thus the laser light is utilized with high efficiency.
By contrast, when a laser beam having a comparatively small width and having an intensity distribution as shown in FIG. 12 is made to converge at a convergence position the same distance away from a shaping element as in the above-described example as a laser beam having the same cross-sectional size and having an even intensity distribution, the obtained intensity distribution is as shown in FIGS. 13A and 13B. In this way, a laser beam having a small width is formed into a laser beam of which the intensity drops gradually in its peripheral portion and which thus exhibits dull intensity variation. Accordingly, an even intensity distribution is obtained only in a small area, and thus the laser light is utilized with low efficiency.
In both of these examples, the shaping element is optimized for the intensity distribution shown in FIGS. 10 and 12 respectively. Even with a shaping element having an optimized shaping characteristic like this, unsatisfactory shaping performance results if the beam width as observed on the shaping element, and thus the numerical aperture there, is small. When a laser beam that diverges in an anisotropic fashion is shaped by the use of a single shaping element, even if the shaping element is an anisotropic one, it is inevitable that shaping performance is unsatisfactory in the direction in which the laser beam has a smaller beam width.
When a laser beam that diverges in an anisotropic fashion is shaped by the use of an isotropic shaping element, the degradation of shaping performance is more striking. An example of the intensity distribution obtained at the convergence position with a shaping element optimized in the direction in which the laser beam has a larger beam width is shown in FIGS. 14A and 14B. In this example, in the direction in which the laser beam has a smaller beam width, there is almost no range in which an even intensity distribution is obtained. Conversely, although not illustrated, when a shaping element optimized in the direction in which the laser beam has a smaller beam width is used, shaping performance is unsatisfactory in the direction in which the laser beam has a larger beam width.
An object of the present invention is to provide a laser radiating optical system that efficiently converts a laser beam that has an uneven intensity distribution and that diverges in an anisotropic fashion into a laser beam having an even intensity distribution on an irradiation target surface.
To achieve the above object, according to one aspect of the present invention, a laser radiating optical system is composed of a laser light source that emits a laser beam having different vertical angles in mutually perpendicular first and second directions and having an uneven intensity distribution, a first shaping optical system, and a second shaping optical system.
The first shaping optical system includes a first collimator lens that forms the laser beam exiting from the laser light source into a parallel beam with respect to the first direction and a first shaping element that converts the intensity distribution of the laser beam exiting from the first collimator lens with respect to the first direction and that simultaneously makes this laser beam converge at a convergence position a predetermined distance away from the laser light source with respect to the first direction. The second shaping optical system includes a second collimator lens that forms the laser beam exiting from the laser light source into a parallel beam with respect to the second direction and a second shaping element that converts the intensity distribution of the laser beam exiting from the second collimator lens with respect to the second direction and that simultaneously makes this laser beam converge at a convergence position a predetermined distance away from the laser light source with respect to the second direction.
The laser beam emitted from the laser light source has an uneven intensity distribution and diverges in an anisotropic fashion. The first shaping optical system acts on this laser beam only with respect to the first direction, and does not affect it with respect to the second direction. Similarly, the second shaping optical system acts on the laser beam emitted from the laser light source only with respect to the second direction, and does not affect it with respect to the first direction. Accordingly, the first and second shaping optical systems can be designed independently of each other. It is possible to design the first and second shaping optical systems in such a way that they make the laser beam converge at identical convergence positions, i.e. that the laser beam is made to converge at a common convergence position in the first and second directions.
By designing the first and second shaping optical systems according to the vertical angles of the laser beam, i.e. the degrees to which the laser beam diverges, in the first and second directions respectively, it is possible to make the laser beam have substantially the same beam width in the first and second directions at the common convergence position and have an even intensity distribution both in the first and second directions at the convergence position. Moreover, it is possible to shape the entire laser beam emitted from the laser light source, and thus make an efficient use of the laser light.
In this laser radiating optical system, if the laser beam emitted from the laser light source is assumed to have a larger vertical angle in the first direction than in the second direction, it is preferable to dispose the first shaping optical system between the laser light source and the second shaping optical system. By converting the intensity distribution first with respect to the first direction, in which the degree of diversion is greater, and then with respect to the second direction, in which the degree of diversion is smaller, it is possible to make the first and second shaping optical systems have substantially equal numerical apertures at their exit sides with respect to the common convergence position, and thereby make them exhibit substantially equal performance in terms of intensity conversion and convergence. This helps make the intensity distribution at the common convergence position substantially the same in the first and second directions and thereby enhance evenness of intensity.
Moreover, it is preferable to design the laser radiating optical system to fulfill the relationship represented by formula (1) below.
0.8xe2x89xa6xcex2xc2x7fL1xc2x7fD2/(fL2xc2x7fD1)xe2x89xa61.25xe2x80x83xe2x80x83(1)
Here, fL1 represents the focal length of the first collimator lens, fL2 represents the focal length of the second collimator lens, fD1 represents the focal length of the first shaping element, and fD2 represents the focal length of the second shaping element. Moreover, assuming that the laser beam emitted from the laser light source has a vertical angle xcex81 in the first direction and a vertical angle xcex82 in the second direction, xcex2 is defined as xcex2=tan (xcex81/2)/tan (xcex82/2).
Formula (1) defines the range of the ratio of the numerical aperture of the first shaping optical system to the numerical aperture of the second shaping optical system on the basis of the difference between the degrees to which the laser beam diverges in the first and second directions. When the central part of formula (1) is equal to 1, the first and second shaping optical systems have equal numerical apertures, and thus the intensity distribution of the laser beam at the common convergence position is the same in the first and second directions. When formula (1) is fulfilled, there is no significant difference between the numerical apertures of the two shaping optical systems, and accordingly the intensity distribution at the convergence position is kept satisfactorily even.
The laser radiating optical system may be additionally provided with an optical system that makes the laser beam having traveled past the convergence position converge once again. This makes it possible to make the laser beam, already made even at the convergence position, have a still smaller beam width while maintaining the evenness thereof.